NEW DELHI — Nearly two dozen crossbred lions are slowly dying in northern India from a mysterious disease afflicting the hybrid offspring of Asiatic and African cats paired in a discontinued experimental program.

Zookeepers are mournfully watching the results of the program, which began in the late 1980s at the Chhatbir Zoo and was ended in 2002 after many of the nearly 80 crossbred lions were struck by a mysterious disease linked to inbreeding and a weakened gene pool, said Kuldip Kumar, Punjab state’s conservator of forests and wildlife.

Wildlife officials had originally hoped the hybrid cats could be introduced into the wild in an effort to bolster India’s endangered wild lion population.

Instead, the offspring found it hard to walk, let alone run, because their hind legs were weak. And by the mid 1990s the big cats -- which live for up to 20 years in captivity -- showed symptoms of failing immune systems.

When the program ended, all of the male lions were given vasectomies to prevent further breeding, Kumar said.

Indian wildlife laws prohibit killing the animals.

It will take about six years for the remaining 22 crossbred lions to die of natural causes, Kumar said.

Zoo authorities have decided to launch a new captive breeding project using “pure Asiatic lion stock from other zoos in the country, but only after the last of the earlier crop of lions have been phased out,” he said.

The zoo has recently built an enclosed area for the oldest and most infirm of the lions, so they are not attacked by the more robust cats.

“At any time the zoo has around four to five lions that are too old and weak to compete with the younger, more aggressive lions,” Kumar said.

The lions are fed boneless meat, he said.

"The effort here is to help them die with dignity," said Dharminder Sharma, a senior zoo official. "We give them all the facilities to live a happy life in their last years."

Wildlife experts say rampant poaching is driving the Asiatic lion to extinction, especially in the Gir forests in western India, the last wild refuge of the big cats.

The last lion census conducted in the forests in 2000 put the number of Asiatic lions at 320. The animals’ numbers have further dwindled due to poaching, open wells that act as death traps and human encroachment on lion habitat.

Lions are killed for their pelts and claws, both of which command a huge price in the illegal wildlife trade.